1. Field of the Invention.
The present invention relates to compositions and methods for treating oil and gas wells to stimulate the production of hydrocarbons therefrom.
2. Description of the Prior Art.
The use of acid to stimulate the production of hydrocarbons from damaged formations and formations having naturally low permeabilities is well known. Typically, the acid is pumped through the well into the formation where it dissolves the damaging material and/or reacts with the matrix and increases the size of pore spaces and channels therein.
When the flow of hydrocarbons is restricted only by formation damage, a large increase in productivity can usually be achieved by merely pumping the acid into the formation. Formation damage typically results from fines migration and/or plugging with substances such as drilling mud, water/oil emulsions and the like. Such damage is usually located in close vicinity to the wellbore. Once the damaging substances are removed, hydrocarbons from all over the formation can flow to the wellbore.
When the flow of hydrocarbons is restricted due to the naturally low permeability of the formation, however, a large increase in productivity is not as easily achieved. Upon entering formations having naturally low permeabilities such as formations consisting of limestone or sandstone, acid rapidly diffuses and begins reacting with the matrix. Delivery of the acid deeply and uniformly into the formation before it spends is often difficult or impossible. Inasmuch as the flow of hydrocarbons in formations having naturally low permeabilities is only increased in those areas to which unconsumed acid is delivered, any increase in productivity is proportional to the distance from the wellbore to which stimulation occurs.
Although the rate of acid spending can sometimes be reduced by increasing the viscosity of the acid fluid, the existence of an undesirable water zone above or below the formation being treated or other conditions making potential fracturing undesirable can prevent the use of an injection pressure sufficient to accommodate the increase in viscosity. Lowering the injection rate to lower the injection pressure negates the benefits of delayed spending that result from the increase in viscosity.
As a partial solution, various acid systems have been developed wherein the rate of acid diffusion and spending is decreased allowing unconsumed acid to be delivered to greater distances within the formation under normal injection rates and pressures. These retarded acid systems include micellar solutions, macroemulsions and microemulsions formed by the addition of various fluids, surface active agents and/or other special chemicals to the acid. Microemulsions are particularly effective due to their ability to restrict the mobility of the dispersed phase, their near zero interfacial tension and the size of the spherical droplets that form the same. They can rapidly penetrate into tight formations. The components forming microemulsions interact to provide a barrier to diffusion of the acid allowing unconsumed acid to be delivered deeply and uniformly into the formation.
Unfortunately, many of the microemulsions and other retarded acid systems developed heretofor are not effective in certain applications. Many of the systems undergo a phase change and separate when exposed to high temperatures or when diluted with water or brine. Some systems tend to form messy emulsions with crude oil which further plug the formation. Other systems are not capable of carrying a sufficient amount of acid into the formation to achieve the desired results.
By the present invention, improved well acidizing compositions and a method of treating a subterranean formation with the same are provided. The inventive compositions provide a low viscosity, retarded acid system that allows a large amount of unconsumed acid to be efficiently delivered deeply and uniformly into a formation. The effectiveness of the system is not diminished by the temperature of the formation, the formation of emulsions or incompatibility of the compositions with dilution water or brine.